Method for using a coated fluoropolymer substrate pellicle in semiconductor fabrication

ABSTRACT

A dust cover having a film with an inorganic, anti-reflective coating and method for use during semiconductor fabrication. The dust cover is primarily for use during photolithography. The dust cover may include an amorphous fluoropolymer film having an inorganic, anti-reflective coating attached to a frame. The inorganic, anti-reflective coating preferably has a refractive index below 1.4.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional from U.S. patent application Ser. No.09/371,823, filed by Joseph S. Gordon on Aug. 11, 1999 and entitled“Dust Cover and Method for Semiconductor Fabrication,” now U.S. Pat. No.6,280,885.

This application is related to U.S. patent application Ser. No.09/772,777, filed by Joseph S. Gordon on Jan. 30, 2001, entitled “DustCover For Use In Semiconductor Fabrication”.

TECHNICAL FIELD OF THE INVENTION

This invention relates in general to the field of semiconductor waferfabrication and, more particularly, to a dust cover having a film withan inorganic, anti-reflective coating and method of use.

BACKGROUND OF THE INVENTION

Semiconductor manufacturing often involves a series of processesincluding deposition, photolithography and etching. During thephotolithographic process, semiconductor manufacturers often use aphotomask to copy an image of an electronic circuit on to asemiconductor wafer. Photomasks come in various sizes and shapes (e.g.,a one times photomask or a reticle, which is a photomask that can beshot several times onto a single wafer with a photolithographic toolknown as a stepper). Photomasks generally include a quartz blank with apatterned chrome layer deposited on one surface of the quartz blank.This patterned metal layer contains a microscopic image of an electroniccircuit.

During the photolithographic process, this image is projected onto awafer. If the image is projected several times onto a single wafer, thephotomask is known as a reticle. As design rules have moved towardsmaller and more dense integrated circuit (IC) devices, the quality ofthe projected image has become increasingly important. A poorlyprojected image may result in the manufacture of a non-functioning ICdevice. In many cases, a dust particle resting on the surface of aphotomask or reticle, causes sufficient distortion of the projectedimage to render the IC device non-functioning.

As such, many, if not most, IC manufacturers rely on a dust cover film,which may on occasion be referred to as a pellicle, to keep dust fromlanding on the surface of the photomask. Typically, the film is attachedto a frame, which may be attached to the photomask such that the film isheld at a fixed distance from the surface of the photomask. During thephotolithographic process, the image of any dust particle resting on thefilm will be out of focus on the wafers surface. As a result, theprobability of a dust induced defect at the wafer surface is reduced.

Ideally, a dust cover film should be invisible to the radiant energy ofthe photolithographic tool. In order to produce clear, well-definedpatterns, the film should preferably transmit nearly 100% of the radiantenergy used. Most films, however, do not transmit 100% of the radiantenergy. A small percentage of the radiant energy is reflected at theinterfaces of the film's surface and air. Generally, the amount of lightreflected at the interface of two transparent layers (e.g., air andfilm) depends upon the refractive index, N, of the two layers. Moreprecisely, the amount of reflection depends upon the difference betweenthe N of the first layer and the N of the second layer. The greater thedifference, the more reflection.

The index of refraction, N, for air is approximately 1.0. A dust coverfilm, on the other hand, may have an N of 1.5 or greater. Thisdifference in index of refraction can result in an average reflection ofapproximately 8 percent of the light striking the surface at a normalincidence (90 degrees to the face of the film). Moreover, taking intoaccount interference within the film, there may be peak reflections ofapproximately 16 percent. As a result, considerably less than 100percent of the radiant energy used during the photolithographic processpasses through the dust cover film.

Conventional techniques for combating this problem involve the additionof a film layer having a lower refractive index to a substrate film,which is usually made of nitrocellulose. The additional film layer isoften spun onto the substrate film. As such, the substrate film needs tobe relatively thick (approximately 1-2 μm) to withstand the forcesassociated with the spinning process. Moreover, when thinner substratefilms are used, damage to the substrate film during the spinning processbecomes more likely.

Other conventional solutions include using a single layer of film with arelatively low refractive index. This conventional single layer approachdoes not allow for producing a dust cover film with a refractive indexmuch below 1.4.

SUMMARY OF THE INVENTION

In accordance with teachings of the present disclosure, a dust coverfilm with an inorganic, anti-reflective coating and method of use aredescribed. The described film and method of use provide significantadvantages over prior technologies.

According to one aspect of the present disclosure a dust cover isdescribed for use during photolithography. The cover includes a frameattached to a fluoropolymer film having an inorganic, anti-reflectivecoating. The frame may be manufactured from various materials including,for example, aluminum. The inorganic, anti-reflective coating preferablyhas a refractive index below 1.3. More preferably, the inorganic,anti-reflective coating has a refractive index between 1.13 and 1.2.

In one embodiment, the coated fluoropolymer film could be an amorphousfluoropolymer. The amorphous fluoropolymer may include, for example,copolymers of 30 to 99 mole percent perfluoro-2,2-dimethyl-1,3-dioxole(PDD) and complementary amounts of at least one comonomer. Preferably,the copolymers will have a glass transition temperature of at least 80°Celsius.

The fluoropolymer film may also include an inorganic anti-reflectivecoating of calcium fluoride (CaF₂). The coating may be applied in anumber of ways, for example, physical vapor deposition. Other inorganicfluorides may also be used. For example, magnesium fluoride (MgF₂) maybe substituted for CaF₂.

One method of manufacturing a coated film for use in a dust coverincorporating teachings of the present invention includes evaporating aninorganic coating material in a vacuum chamber and depositing it on afluoropolymer film. The pressure in the chamber may be increased abovenormal to lower the refractive index of the resulting inorganicanti-reflective coating. A good approximate starting pressure may be1×10⁻⁴ torr. The factors affecting the appropriate pressure include,among others, source and type of fluoride, size and type of film, rateof deposition and desired refractive index.

The technical advantages of a dust cover incorporating teachings of thepresent invention include an increased transmission percentage as wellas an increased durability (i.e., longer life cycle). These advantagesmay help improve efficiencies during a photolithographic process. Inaddition, the disclosed dust cover should have improved transmissionacross a broad spectrum of frequencies.

Another technical advantage of the present invention arises duringmanufacture of the dust cover itself. Conventionally, an anti-reflectivecoating is placed in solution and spun onto a substrate film. Thedynamics of this systems as well as the solvents used when creating thesolutions often damage the substrate film. An inorganic anti-reflectivecoating may be applied via physical vapor deposition, which should helpreduce manufacturing problems.

Other technical advantages will be apparent to one of ordinary skill inthe art in view of the specification, claims, and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and advantagesthereof may be acquired by referring to the following description takenin conjunction with the accompanying drawings, in which like referencenumbers indicate like features, and wherein:

FIG. 1 shows a cross-sectional view of a photomask with a dust coverincorporating teachings of the present invention mounted thereto;

FIG. 2A shows a graph relating the percentage of light transmittedverses the wavelength of light transmitted for a fluoropolymer filmwithout an inorganic, anti-reflective coating;

FIG. 2B shows a graph relating the percentage of light transmittedverses the wavelength of light transmitted for a fluoropolymer film withan inorganic, anti-reflective coating;

FIG. 3 shows a graph relating the percentage of light transmitted atapproximately 248 nanometers to the amount of radiant energy usedmeasured in jewels per centimeter squared for an amorphous fluoropolymerfilm with an inorganic anti-reflective coating; and

FIG. 4 shows an enlarged cross-sectional view of a dust cover filmincorporating teachings of the present invention including an amorphousfluoropolymer substrate coated with inorganic, anti-reflective coating.

DETAILED DESCRIPTION

Preferred embodiments of the present invention and their advantages arebest understood by reference to FIGS. 1 through 4, where in like numbersare used to indicate like and corresponding parts.

FIG. 1 shows a cross-sectional view of a photomask 10 having a patternedlayer 12, which may be formed from a variety of materials (e.g.,chrome). Patterned layer 12 forms a microscopic image of a circuit thatmay be projected onto a wafer. FIG. 1 shows a dust cover, indicatedgenerally at 14, incorporating teachings of the present disclosure. Dustcover 14 may be coupled to Photomask 10 such that dust particles,figuratively represented at 16, alight on dust cover 14 as opposed tophotomask 10. When coupling dust cover 14 to photomask 10, receivingsurfaces 26 may rest upon photomask 10.

Photomask 10 may have a variety of sizes and shapes. For example,photomask 10 may be round, rectangular or square. Photomask 10 may alsobe any of a variety of photomask types. For example, photomask 10 may bea one time master, a five inch reticle or a six inch reticle.

As depicted in FIG 1, dust cover 14 includes a frame 18, which may beformed of aluminum, and a film 20 attached to frame 18. Film 20 mayinclude a coated substrate film of fluoropolymer. In preferredembodiments, the substrate film may include an amorphous fluoropolymercoated with an inorganic anti-reflective coating.

Film 20 provides a surface upon which dust particles 16 may alight. Assuch, dust particles 16 remain a defined distance away from photomask10. This may be especially important during the photolithographicprocess of IC manufacturing. During photolithography, photomask 10 anddust cover 14 are exposed to radiant energy, indicated generally at 22,produced by a photolithographic tool, figuratively represented at 24.Radiant energy 22 may include light of various wave lengths, forexample, deep ultra violet light.

In operation film 20 maintains dust particles 16 a defined distance fromphotomask 10 and allows a large percentage of radiant energy 22 to passthrough it. As such, dust particles 16 will likely be out of focus atthe surface of the wafer being shot and the exposed image should beclear and well-defined.

The graph of FIG. 2A shows the percentage of light transmitted through afluoropolymer film with no coating verses the wavelength of light,measured in nanometers. The film tested to produce FIG. 2A transmittedapproximately 98.7% of light having a wavelength of 306 nanometers.

The graph of FIG. 2B shows the percentage of light transmitted through acoated film incorporating teachings of the present invention verses thewavelength of light, measured in nanometers. The coated film tested toproduce FIG. 2B had an amorphous fluoropolymer substrate and aninorganic anti-reflective coating. At wavelengths of approximately 306nanometers, the coated film of FIG. 2B transmitted over 99% of theradiant energy. In addition to allowing more radiant energy to transmitthrough the coated film, the coated film may also prove to be moredurable than non-coated film.

FIG. 3 shows a graph depicting the percentage of light transmittedthrough a dust cover film incorporating teachings of the presentinvention at wavelengths of approximately 248 nanometers. The FIG. 3graph plots light transmission for different amounts of radiant energydoses measured in jewels per centimeter squared. The film used toproduce the graph of FIG. 3 had an amorphous fluoropolymer substrate andan inorganic anti-reflective coating.

FIG. 4 shows an enlarged cross-sectional view of a coated fluoropolymerfilm, depicted generally at 30, incorporating teachings of the presentdisclosure. Film 30 includes an amorphous fluoropolymer substrate 32coated with an inorganic, anti-reflective coating 34. Substrate 32 has atop surface 36 and a bottom surface 38. In the depicted embodiment ofFIG. 4, substrate 32 has a coating 34 on both top surface 36 and bottomsurface 38.

As depicted in FIG. 4, coated fluoropolymer film 30 includes anamorphous fluoropolymer substrate 32. The amorphous fluoropolymer ofsubstrate 32 may include, for example, copolymers of 30 to 99 molepercent perfluoro-2,2-dimethyl-1,3-dioxole (PDD) and complementaryamounts of at least one comonomer. Preferably, the copolymer will have aglass transition temperature of at least 80° Celsius. The at least onecomonomer may be selected from the class consisting of the followingcompounds:

(a) tetrafluoroethylene,

(b) cholorotrifluoroethylene,

(c) vinylidene fluoride,

(d) hexafluoropropylene,

(e) trifluoroethylene,

(f) perfluoro (alkyl vinyl ethers) of the formula CF₂═CFOR_(F), whereR_(F) is a normal perfluoroalkyl radical having 1-3 carbon atoms,

(g) fluorovinyl ethers of the formula CF₂═CFOQZ, where Q is aperfluorinated alkylene radical containing 0-5 ether oxygen atoms,wherein the sum of the C and O atoms in Q is 2 to 10; and Z is a groupselected from the class consisting of —COOR, —SO₂F, —CN, and —OCH₃,where R is a C₁-C₄ alkyl,

(h) vinyl fluoride, and

(i) (perfluoroalkyl) ethylene, R_(f)CH═CH₂, where R_(f) is a C₁-C₈,normal perfluoroalkyl radical.

As used herein, the term “complementary” means that the mole percentageof PDD plus the mole percentages of any or all of the above comonomers(a) through (i) that are present in a copolymer add together to equalapproximately 100%.

The comonomers may also have preferred upper limits regarding theirrespective mole percentages. For example, preferred upper molepercentage limits, M_(a) . . . M_(i), for the above-listed comonomersmay be as follows:

(a) for tetrafluoroethylene, M_(a)=70,

(b) for chlorotrifluoroethylene, M_(b)=70,

(c) for vinylidene fluoride, M_(c)=70,

(d) for hexafluoropropylene, M_(d)=15,

(e) for trifluoroethylene, M_(e)=30,

(f) for CF₂═CFOR_(f), M_(f)=30,

(g) for CF₂═CFOQZ, M_(g)=20,

(h) for vinyl fluoride, M_(h)=70, and

(i) for R_(f)CH═CH₂, M_(i)=10;

In another embodiment, an amorphous fluoropolymer may include more thanone comonomer. Amorphous fluoropolymers with more than one comonomerwill preferably limit the amount of each comonomer present in thecopolymer such that the sum, S, of the ratios of the actual molepercentages, m_(a) . . . m_(i), relative to the corresponding preferredupper limit mole percentages, M_(a) . . . M_(i), is no larger than 1, asshown below:

S=m _(a) /M _(a) +m _(b) /M _(b) + . . . +m _(i) /M _(i)≦1.

Anti-reflective coating 34 may be applied as a single layer coating (asdepicted in FIG. 4). When applying a single layer coating, the followingequation may help define the preferred index of refraction for thesingle layer anti-reflective coating:

N_(ar)=N_(s)

N_(ar)=index of refraction for anti-reflective coating

N_(s)=index of refraction for substrate film.

For example, a fluoropolymer substrate film for use with deep ultraviolet photolithography may have a refractive index of 1.29 to 1.44. Assuch, a single layer anti-reflective coating for this fluoropolymersubstrate may have a preferred index of refraction between 1.13 and 1.2.

As an alternative to a single layer coating, multiple layers of coatingmay be applied to substrate 32. Each of the multiple layers may have aslightly different refractive index. For example, a three coating systemmay be employed. The first coat may, for example, have a reflectiveindex of 1.3, which is relatively close to a typical refractive indexfor amorphous fluoropolymer film (i.e., approximately 1.4). The secondcoating may have a reflective index of 1.2, and the third coat may havea reflective index of 1.1, which is relatively close to the refractiveindex of air (i.e., approximately 1.0). As such, the difference inrefractive index between any two layers would be approximately 0.1. Thiswould equate to nearly 100% transmission of radiant energy during thephotolithographic process.

Although the disclosed embodiments have been described in detail, itshould be understood that various changes, substitutions and alterationscan be made to the embodiments without departing from their spirit andscope.

What is claimed is:
 1. A method for fabricating a semiconductor device,comprising: placing an inorganic anti-reflective coating on a film, theantireflective coating having a refractive index less than or equal to1.2; attaching the film to a same to form a dust cover; coupling thedust cover to a photomask; and exposing the dust cover and the photomaskto radiant energy.
 2. The method of claim 1, wherein the radiant energycomprises deep ultra violet radiation.
 3. The method of claim 1, furthercomprising: removing the film from the frame; and attaching a new filmto the frame.
 4. A method for fabricating a semiconductor devicecomprising: applying an inorganic anti-reflective coating to afluoropolymer substrate having a top and bottom surface, theanti-reflective coating having a refractive index less than or equal to1.2; coupling the coated fluoropolymer substrate to a frame operable tosupport the coated fluoropolymer substrate to for a dust cover; restingthe dust cover on a reticle; positioning a semiconductor wafer proximatethe reticle; and applying radiant energy to the semiconductor waferthrough the dust cover and reticle.
 5. The method of claim 4 furthercomprising: applying a plurality of adjoining inorganic anti-reflectivecoatings to the fluoropolymer substrate; and each adjoining inorganicanti-reflective coating having a different refractive index.
 6. Themethod of claim 5 further comprising differing the refractive index ofadjoining inorganic anti-reflective coatings by approximately 0.1. 7.The method of claim 4 further comprising the fluoropolymer substratehaving a refractive index between 1.29 and 1.44.
 8. The method of claim4 further comprising the inorganic anti-reflective coating having arefractive in ex between 1.13 and 1.2.
 9. The method of claim 4 furthercomprising the fluoropolymer substrate consisting of an amorphousfluoropolymer having between 30 and 99 mole percentperfluoro-2,2-dimethyl-1,3-dioxole and complementary amounts of at leastone comonomer.
 10. The method of claim 4 further comprising theinorganic anti-reflective coating including calcium fluoride.
 11. Themethod of claim 4 further comprising applying the inorganicanti-reflective coating to the top surface and the bottom surface of thefluoropolymer substrate.
 12. The method of claim 4 further comprisingthe thickness of the fluoropolymer substrate having a thickness ofapproximately 0.8 μm.
 13. A method for fabricating a semiconductordevice comprising: coupling a fluoropolymer substrate coated with aninorganic anti-reflective coating having a refractive index less than orequal to 1.2 to a frame to form a dust cover; resting the dust cover ona photomask; positioning a semiconductor wafer proximate the photomask;and exposing the semiconductor wafer to radiant energy.
 14. The methodof claim 13 further comprising forming the fluoropolymer substrate froman amorphous fluoropolymer having between 30 and 99 mole percentperfluoro-2,2-dimethyl-1,3-dioxole and complementary amounts of at leastone comonomer and the fluoropolymer substrate having a refractive indexbetween 1.29 and 1.44.
 15. The method of claim 13 further comprisingexposing the semiconductor wafer to deep ultra violet radiation throughthe dust cover and photomask.
 16. The method of claim 13 furthercomprising forming a plurality of adjoining inorganic anti-reflectivecoatings on the fluoropolymer substrate.
 17. The method of claim 16further comprising forming the adjoining inorganic anti-reflectivecoatings with a difference of approximately 0.1 in value of therespective refractive indices of the adjoining inorganic anti-reflectivecoatings.
 18. The method of claim 13 further comprising forming theinorganic anti-reflective coating from material having a refractiveindex between 1.13 and 1.2.
 19. The method of claim 13 furthercomprising forming an inorganic anti-reflective coating on both a tosurface and a bottom surface of the fluoropolymer substrate.
 20. Themethod of claim 13 further comprising replacing the coated fluoropolymersubstrate after exposing the semiconductor wafer.